The present invention is in the field of electron beam recording, and particularly relates to a silver halide-based electron beam writing medium generally useful for storing information and particularly useful for storing patterns which can provide optical masks for microcircuit fabrication or the like. The films incorporated in the present media are related in composition to the films described in our concurrently filed co-pending patent application, Ser. No. 86,690, disclosing ultraviolet light-sensitive films for optical information storage media.
The use of evaporated silver halide films as photographic media is well known. A recent review of the patent literature in this field is provided by U.S. Defensive Publication T966,003 by J. E. Maskasky, dated Jan. 3, 1978. U.S. Pat. No. 3,219,448 is typical of such literature, describing the vacuum deposition of a silver halide film on a substrate such as glass, the exposure of the film to light to provide a latent image therein, and the development of that latent image by chemical means to provide a visible image in the film.
The use of evaporated layers of silver halide or other metallic photosensitive compounds as recording media for images produced by electron beam exposure has received more recent attention. U.S. Pat. No. 3,664,837 to Stanley suggests the use of such layers to provide conductor lines on microcircuit chips, while the article "Evaporated Silver Bromide as an Electron Beam Recording Material" by A. Shepp et al. in Photographic Science and Engineering, 11, (5), pp. 322-328 (1967) discusses the various factors governing the behavior of such films during electron bombardment and subsequent development.
Generally, films such as described in the foregoing publications are used in the photographic mode, which means that a chemical development step is required to develop the latent image formed in the film on light or electron beam exposure, in order to provide a silver image with useful optical density. An electron beam storage medium which could provide a visible image when used in a direct writing mode, meaning a mode wherein the image would be produced by electron bombardment without any requirement for chemical development, would be highly desirable.
Photographic films of the kind above described typically exhibit relatively limited resolution when used for electron beam information storage. For applications such as microcircuit masking, image resolution on the order of about 1 micron (1000 lines/mm) or less are required for very large scale integrated circuit manufacture. The electron-beam-written films reported by Stanley and Shepp et al. above reportedly provide resolutions in the 200-300 lines/mm range.
At present, high resolution optical masks for microcircuit fabrication are made using glass-supported chromium film mask blanks by applying a layer of an organic electron-beam-writable photoresist over the chromium film, writing a masking pattern onto the resist with an electron beam, "developing" the selectively exposed resist by removing the exposed (or unexposed) portions thereof, and then chemically removing the physically exposed regions of the chromium film to provide the final optical masking pattern therein. A discussion of the behavior and use of polymeric resists in microcircuit fabrication is provided by M. J. Bowden in "Electron Irradiation of Polymers and Applications to Resists for E-Beam Lithography", CRC Critical Reviews in the Solid State Sciences, pp. 223-264 (February 1979). The use of silver halide emulsions as resists has recently been described by R. B. Marcus et al. in the Technical Digest of the IEEE International Election Devices Meeting (December 1978) pages 591-593.
While optical microcircuit masks provided in accordance with procedures such as above described can provide sufficient resolution for present commercial microcircuit products, the large number of processing steps required for the production of such masks is clearly undesirable. A medium into which a high resolution masking pattern of useful optical density could be directly written without any requirement for chemical development would offer a distinct processing advantage.
It is accordingly a principal object of the present invention to provide a medium suitable for electron beam recording which does not require a development step for the production of a visible image, and which exhibits high resolution at presently attainable writing energy levels.
It is a further object of the invention to provide a method for making an optical mask, useful for microcircuit fabrication or the like, wherein an electron beam recording medium of enhanced sensitivity is directly darkened by selective exposure to an electron beam to provide a masking pattern of useful optical density and high resolution.
Other objects and advantages of the invention will become apparent from the following description thereof.